1. Field of the Invention
An embodiment of the present invention relates to the microelectronics field. More specifically, the present invention relates to integrated capacitors.
2. Discussion of the Related Art
Capacitors are components commonly used in most electronic circuits. Referring in particular to an electronic device being integrated in a chip of semiconductor material, many techniques are known for making the capacitors in the same chip wherein there are made other functional components of the integrated device, being both passive (such as resistors) and active (such as transistors).
For example, (integrated) Metal-Oxide-Metal (MOM) capacitors include two plates of conductive material (typically metal) between which there is grown a thin oxide layer of the metal (having insulating properties) to form a dielectric layer of the capacitor. The MOM capacitors are nowadays the preferred choice in many types of integrated devices for their manufacturing simplicity, even if they need particular manufacturing precautions in order not to damage the thin oxide layer.
On the contrary, (integrated) Metal-Insulator-Metal (MIM) capacitors are formed by two plates of conductive material (typically metal) between which there is deposited a layer of insulating material (for example, Silicon Nitride) to form the dielectric layer of the capacitor. The Silicon Nitride has a high dielectric constant, so that the MIM capacitors have a specific capacitance (per unit area of their plates) being greater than that of the MOM capacitors; this allows making more compact capacitors that, for the same overall capacity, occupy a lower area of the chip than the MOM capacitors do. Furthermore, the MIM capacitors allow integrating other components below them, with a further saving of area of the chip.
Typically, each MIM capacitor is made by forming a bottom plate thereof from a portion of a metal layer of the chip, being commonly used for distributing signals and/or power supply among functional components of the integrated device, while an upper plate thereof is formed within a protective layer (of insulating material) protecting the metal layer wherein there are made the bottom plate and the dielectric layer being formed thereon.
However, in the present state of the art, the quality of the dielectric layer of the MIM capacitors is not optimal. In fact, the layer of insulating material from which there is formed the dielectric layer is typically obtained through a deposition process on the bottom plate of the MIM capacitor; such deposition should be performed at temperatures such that the bottom plate is not damaged. Since the metal layer (from which the bottom plate is obtained) that are typically used in the integrated devices has a relatively low melting temperature (usually, not grater than 400° C.), the dielectric layer has to be deposited necessarily at process temperatures that are not optimal (for example, 200° C.). This has a negative effect on the chemical and mechanical properties of the dielectric layer, such as the planarity, with a consequent worsening of its insulating properties.
Furthermore, the low melting temperature of the bottom plate turns out to be particularly critical in power applications; particularly, when the MIM capacitors are integrated together with power components, the bottom plate should have a well-defined thickness depending on the requirements of the power components. For example, in the case of bipolar-CMOS-DMOS (BCD) technology—where BJTs for precision analog applications, CMOS for digital applications, and DMOS for power applications are integrated in the same chip—the metal layer (and thus the bottom plate of the capacitor) has a typical thickness of 0.7-1.0 μm (for limiting an ON-resistance of the power components and for reducing electro-migration phenomena). If the bottom plate has a low melting temperature, it is more subject to thermo-plastic deformations being due to thermal stresses during the operations following its manufacturing; this involves a thick-grained structure of the bottom plate, which causes kinks and surface irregularities that affect not only the performance of the capacitor (such as a voltage breakdown thereof) but also the performance of the power components.